1. Field of the Invention
The present invention relates to a high pressure metal vapor lamp.
2. Description of the Prior Art
Generally, high pressure metal vapor lamps include an outer protective envelope, an arc tube supported therein and containing a discharge sustaining filling, appropriate electrodes operatively associated with such filling and electrically connected to a base member which connects the lamp to a source of electrical power, and gettering means within the protective envelope. During the use of such a high pressure metal lamp it is necessary for the arc tube to operate at a suitable high temperature in order to obtain the high efficiency which such lamps are capable of achieving. Such operating temperature can be as high as about 1250.degree. C. In view of such a high temperature the lamp needs to have a high degree of vacuum within the protective envelope, which forms the outer jacket, in order to conserve heat and insure optimum operating conditions. The degree of evacuation required is not readily achieved during high volume production, and therefore it is necessary to provide an auxiliary means for obtaining the degree of vacuum required above that which is obtained using the high volume production processing equipment presently available. To this end, an evaporable getter, or a non-evaporable getter of the type incorporated within the outer jacket, are typically provided to remove residual gases that remain after conventional evacuation by pumping.
In addition to removing residual gases during initial gas processing, getters contribute to the longevity of the lamp since getters continue gettering throughout the life of the lamp thereby providing thermal insulation effected by the vacuum in the outer jacket. In addition, continued gettering serves to entrap gases that are outgassed or emitted from internal lamp components and from the heated outer jacket long after initial lamp processing. This is very important, since hydrogen or oxygen liberated from some of the constituents of the outer jacket can react with the arc tube electrical feedthrough, causing premature seal failure.
There are various problems associated with getter processing or activation. For example, one known non-evaporable type of getter requires a heat activation treatment in the range of 850.degree. C. to 900.degree. C. for 25 to 35 seconds and further requires getter placement within the outer jacket of the lamp in a region having temperatures in excess of 250.degree. C. Alternatively, the conventional evaporable Barium flash-type getter is typically in the form of a BaAl4+Ni mix held within an open ring substructure which must be heated to 900.degree. to 1000.degree. C. for a couple of seconds in order to initiate the reaction which liberates the barium for deposition on the wall(s) of the lamp jacket. The evaporable and non-evaporable getters traditionally have used a high frequency heat induction method which couples the induction coils to the getter to perform the required heating. This process is not encumbered owing to the transparent nature of the glass outer jacket. However, the deposited barium film is opaque in nature, making it necessary for the getter to be positioned near the base of the lamp so that the deposition will not interfere with the light output. For example, heretofore, getters of the type discussed above have been used with conventional clear outer jackets or those having an inner surface diffusing coating. Such lamps are of a linear construction by which is meant that their components are stacked in a linear fashion including base, flare stem, elongated arc tube, spring mount, etc. In essence, the arc tube extends along an axis of the lamp which axis extends from the base towards the lens. U.S. Pat. Nos. 4,197,480, 4,333,032, and 4,467,238 describe sodium lamps of this type wherein the arc tube extends along such an axis. The type of getter typically used in this type of lamp is the barium flash getter, and the lamp configuration necessitates that the getter be located in a region near the base of the lamp as described in U.S. Pat. No. 4,333,032. Such positioning of the getter assures that the flashed non-transparent film will have a minimal effect upon light output. In the lamp of U.S. Pat. No. 4,333,032 the barium flash serves as a dominant source for photoelectron emission. However, such an emission causes sodium migration from the arc tube, the depletion of sodium in this manner tending to eventually lead to arc tube darkening and premature lamp failure.
Deviation from the conventional linear type of construction to a more compact unit having an aluminized reflector might be considered. However, it is believed that obstacles relating to such a lamp include (1) an inability to couple the high frequency induction heating to the getter when the getter is enclosed in a sheath of aluminum, and (2) the coupling of the aluminum coating with the high frequency energy causes heating and vaporization of the aluminum coating thereby causing a loss of reflective coating and a redeposition of the vaporized aluminum onto the arc tube and lens. If it were possible to flash a barium-type getter in such a lamp, the getter would be exposed to direct arc tube radiation and the attendant undesirable effects.
In one known type of high pressure sodium lamp having an aluminized reflector, an axially mounted arc tube is provided having a non-evaporative getter on opposite sides of the arc tube. Since it is preferred to provide high pressure sodium lamps with a barium flash getter, the use of the non-evaporative getter in this embodiment appears to be a compromise in gettering performance. In addition, the foregoing lamp design includes a costly dichroic reflective coating. As discussed in U.S. Pat. No. 4,197,480, a dichroic coating dissipates radiation out the back of the reflector thereby decreasing the lens operating temperature to the required level. However, such a coating also allows a considerable amount of visible light to escape.
An object of the invention is to provide an improved high pressure metal vapor lamp. Another object of the invention is to provide a high pressure metal vapor lamp including an evaporable barium getter which can be positioned other than at the base of the lamp without having the deposited barium film interfere with the light output. A further object of the invention is to provide a high pressure metal vapor lamp including an evaporable barium getter wherein sodium migration from the arc tube of the lamp is minimized, if not eliminated. A still further object of the invention is to provide a more compact high pressure metal vapor lamp having an aluminized reflector and including a barium flash getter which is not exposed to direct arc tube radiation and the attendant undesirable effects. Another object of the invention is to provide a high pressure metal vapor lamp which does not require the use of a dichroic coating to decrease the lens operating temperature.